The interior of St. Magnus Martyr is to my mind one of the finest among Wren's interiors. See The Proposed Demolition of Nineteen City Churches (P. S. King & Son, Ltd.).
In London's earlier years more of the central area was residential and there were a number of churches built for the use of the residents. As time went by, this area developed into a mostly commercial area now known as "The City." Attendence and use of the churches dropped off and some were proposed for demolition. Eliot, who worked in the area when employed at Lloyd's Bank (1917-1925) would visit these churches. His note for line 264 may have been a way to get more people involved in saving them.
Sorry, I do not have a link for, or a copy of, The Proposed Demolition of Nineteen City Churches